1. Field of the Invention
The present invention relates to a digital signal reproducing device and, more particularly, to a wave shaping circuit for correcting the waveform of the digital signal, pulse coded signal, or read out from a recording medium, such as a magnetic member.
2. Description of the Prior Art
Recently, a development has been made on an analog signal recording/reproducing system such that the analog signal, such as an audio signal, is first converted to a digital signal. Then, the converted digital signal is modulated, by a pulse code modulation, to a self-clocked type digital signal. The modulated digital signal is recorded on a recording medium, such as a magnetic tape. A reproducer for reproducing the recorded signal is shown in FIG. 1. The pulse code modulation may be any known modulation, such as a 3 PM (three position modulation), or a MNRZI (modified non-return to zero inverted).
In FIG. 1, reference number 1 is a magnetic head, 2 is an AC coupling, 3 is a pre-amplifier, 4 is an equalizer, 5 is a comparator, 6 is an analog PLL circuit and 7 is a digital signal processing circuit. In the case of a multi-channel type recording/reproducing system, the circuit shown in FIG. 1 would be responsive to only one channel of a multi-channel system. A duplicate circuit would be needed for each additional channel used so that the number of circuits would equal the number of channels.
As shown in FIG. 2, the waveform a represents the signal recorded in the magnetic tape. The waveform b represents the signal produced from equalizer 4. The reproduced signal of waveform b carries an unwanted DC component, as indicated by the dotted line c, caused by various factors such as the drooping observed after the AC coupling 2. When the signal (waveform b) is compared with a constant DC level (waveform d), comparator 5 produces a pulse signal shown by waveform e, which is different from the original signal (waveform a) as recorded in the tape. Therefore, the problem is that the signal processed in the stage after comparator 5 will not represent the original signal.